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History revisited

TEXT SASCHA BORRÉE

PHOTOS JAN VETTER

125 years after German engineer Otto Lilienthal built his “normal gliding apparatus,” an exact replica has been developed to explore the dawn of flight

Turn up the wind full blast and the glider will disintegrate. So the testers take it slowly, blowing only a puff of air at the fragile contraption. The displays in the wind tunnel’s control center read three meters per second – just over 10 kilometers an hour. “We eventually want to turn it up to 36 km/h,” says mechanical engineer and aviation technology specialist Henning Rosemann, 60, of the German Aerospace Center (DLR). Europe’s largest wind tunnel, here in Marknesse in the Netherlands, is a 320-meter-long, steel-lined tube crammed with high-tech equipment. But today, the tunnel is being used as a kind of time machine. Inside, attached to a robotic arm, is an ancient-looking flying machine that resembles a huge bat. Its wings are a tracery of wooden ribs covered with a light-colored cloth and stabilized by a graceful tailplane. This is the “normal gliding apparatus” developed by Otto Lilienthal in 1893 – or rather, a painstaking reconstruction of the pioneering aviator’s glider.

“What we usually test here are trucks that require aerodynamic optimization and jet planes that don’t even exist yet,” says project engineer Sascha Heinrich, 48. Often enough, he turns the wind up to double hurricane force when he’s studying modern profiles and surfaces. “But it’s a very different story with this old glider, it puts everything else in the shade.” The gleam in his eyes betrays his excitement at fulfilling a childhood dream. “At 12, while other kids were reading Wild West stories by Karl May, I was devouring everything that Lilienthal wrote.” Lilienthal was not a storyteller; he was the man who made the age-old dream of flight come true. Born in Anklam, a North German town, in 1848, he is said to have begun experimenting at the age of 19 with strange “apparatuses” that could lift off the ground. In 1891, Lilienthal completed the world’s first flight by gliding 15 meters through the air.

“That was exactly 125 years ago,” says Rosemann, glancing at the displays. The wind is now blowing slightly harder at four meters per second, and the tailplane begins to flutter. “Lilienthal’s glider has never been studied properly, despite being a milestone in aviation history.” Just how stable is it? How well did it fly? What was it like to steer? And most importantly: Why, after countless flights, did Lilienthal crash?

Lilienthal’s glider has never been studied properly

Henning Rosemann, aircraft engineer

In a bid to find the answers, Rosemann and the DLR team commissioned a replica of Lilienthal’s most famous glider: the world’s first series-produced flying machine, nine of which Lilienthal was able to sell. Some of these are still around today, but they hang in museums unwilling to loan them for risky experiments. “So we had to have an exact reproduction,” Rosemann explains. The DLR found just the people for the job at the Otto Lilienthal Museum in Anklam, where several reconstructions of the aviation pioneer’s flying apparatuses had been made, “but never with the aim of reproducing the orginal flight characteristics,” says the museum’s director, Bernd Lukasch, 62. The construction team hired a specialist for historical textiles to recreate the English shirt material that Lilienthal used for the wings, but when it came to the wooden frame, they compromised, choosing tropical abachi wood instead of the original willow. “Abaci is easier to work with, but has the same properties,” explains Lukasch.

The glider’s ribcage took shape over the winter of 2015/16 and then the fabric was stretched across it. After two months, the featherlight (20-kilogram) glider was completed. What then? Weren’t the researchers tempted to climb on board and take a running jump off the nearest hill? “Certainly not!” says Rosemann. “This is the glider with which Lilienthal crashed!”

So the researchers placed the fragile vehicle in the wind tunnel for their purely scientific investigation, and a life-size dummy in the pilot’s harness. Lilienthal’s “double” is even dressed like him: vest, knickerbockers, and knee-length argyle socks. Everyone steps outside the tunnel and the wind is switched on. Rapt, they gaze at the displays. These are magical minutes – a pioneering feat in a time machine.

Lilienthal reputedly flew in headwinds of up to 36 km/h. “Our glider should be able to manage that,” says Rosemann. But no one knows for certain. The German Hang Gliding Association once put a Lilienthal reconstruction on a dynamometer to generate headwind, and a wing spar promptly snapped. So the men in the control center are very careful to increase the wind to five, then six meters per second very gradually.

Curve charts appear as a monitor transmits the image from inside the tunnel. Rosemann is silent. He only reacts once the wind has been switched off at the end of the first test series, and then he’s euphoric. “Fantastic! That’s fantastic!” The team rushes down to inspect the glider. Has anything worked loose? No, everything is fine. So: more wind, more load. The robotic arm turns the 6.7-meter wings more steeply into the wind, lowers and turns the glider, simulating different flight movements. They turn up the wind again. Eight, nine meters per second.

The tailplane flutters, then flaps wildly to and fro. But Rosemann just says calmly, “It’s looking good.” Soon, the wind reaches 10 meters per second (36 km/h), the maximum load Lilienthal had to cope with. The glider passes the test and the experts are pleased. “The apparatus was absolutely perfectly designed, it is intrinsically stable.“ Intrinsically stable aircraft automatically regain their balance after gusts of wind or steering errors, which is an important requirement for safe flying. This means that a design error was not the cause of Lilienthal’s fatal accident. Back at his institute, Rosemann analyzes other data and soon identifies the glider’s limitations: “When the nose angle is too steep, the glider becomes uncontrollable.” It looks likely that this, triggered by an upwind, caused the crash. “Given the weather conditions on that day, Lilienthal should never have taken off,” Rosemann says. He is sure that the glider is otherwise safe to fly, and that sets him and his colleagues dreaming. “We had never planned an actual flight, but these results make it feasible.”

No one has yet been selected to actually fly the ancient glider, but there is no dearth of candidates, as Rosemann says: “Test pilots have been contacting us from all over the world.”